Add a compiler_flag rule that exposes the value of the "compiler" fie…

…ld of CROSSTOOL

RELNOTES: None
PiperOrigin-RevId: 198295290

tools/cpp/BUILD

@@ -392,3 +392,7 @@ filegroup(
     name = "crosstool_lib",
     srcs = ["crosstool_lib.bzl"],
 )
+
+load(":compiler_flag.bzl", "compiler_flag")
+
+compiler_flag(name = "compiler")

tools/cpp/compiler_flag.bzl

@@ -0,0 +1,28 @@
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Rule that allows select() to differentiate between compilers."""
+
+load("//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
+
+def _compiler_flag_impl(ctx):
+  toolchain = find_cpp_toolchain(ctx)
+  return [config_common.FeatureFlagInfo(value = toolchain.compiler)]
+
+compiler_flag = rule(
+    implementation = _compiler_flag_impl,
+    attrs = {
+        "_cc_toolchain": attr.label(default=Label("//tools/cpp:current_cc_toolchain")),
+    }
+)

tools/cpp/toolchain_utils.bzl

@@ -34,6 +34,7 @@ def _make_legacy_toolchain(ctx):
         ar_executable = _get_cpp_toolchain_attr(ctx, "ar_executable"),
         strip_executable = _get_cpp_toolchain_attr(ctx, "strip_executable"),
         ld_executable = _get_cpp_toolchain_attr(ctx, "ld_executable"),
+        compiler = _get_cpp_toolchain_attr(ctx, "compiler"),
     )
 
 def find_cpp_toolchain(ctx):